A resist is a layer of a material that adheres to a substrate during semiconductor fabrication. A pattern of openings is formed in the resist through which the substrate is etched or impurities are implanted. The pattern in the resist may be formed, for instance, by exposing certain portions of the resist to ultraviolet (UV) light and etching with a suitable chemical. Sizing control of resist openings or geometries during integrated circuit ("IC") manufacturing is therefore essential to the success of product fabrication.
As circuit complexity is increased, either the size of the ICs or the density of the circuitry must increase. For a number of reasons, small device size is desirable. circuit density and resist geometry size must therefore be minimized. This decrease in geometry size has put a strain on linewidth control efforts. For a smaller geometry, the same sizing variation will have a proportionately greater effect.
Many factors in the pattern transfer process can adversely affect the sizing of geometries. These include resist coating thickness, imaging tool conditions, temperature and concentration of developer, and the etching equipment conditions, among others. Even if all the factors which influence sizing are constantly monitored and adjusted to bring them into control, the final pattern size itself must be measured before further processing to assure that the desired sizing is achieved.
Sizing measurement to monitor and control resist geometries is done using two techniques. The first technique uses an optical instrument to measure the linewidth and geometry of a resist. The resolution of this type of instrument is limited by the wavelength of visible light to around 1.2 micrometers. Optical linewidth measurement is also sensitive to substrate conditions and to the edge profile of the resist geometries being measured.
Where the geometry needed to be measured approaches 1 micrometer, most fabrication facilities find it necessary to use a second technique. There, a scanning electronic microscope (SEM) is used to precisely monitor critical resist dimensions. Recent advances in SEM technology make it now possible to efficiently use SEMs in production lines without destroying the measured product.
Although SEMs can measure geometries smaller than one micrometer, a charging phenomenon increases the uncertainty of the ultimate measurement. This charging phenomenon arises because the number of electrons emitted from the SEM and impinging upon the surface of the measured substrate is greater than the number of electrons emitted from the substrate. This ultimately degrades the focusing ability and hence the resolution of the SEM. The charging problem is aggravated when a nonconductive resist pattern lies on top of a conductive substrate and after baking.
Several techniques have been developed to minimize the electron charging phenomenon. The phenomenon can be minimized by using a lower electron beam current, by tilting the substrate with respect to the beam, and by adding water or other chemicals to the resist. Fewer electrons penetrate into the substrate at lower currents but fewer electrons will be present to illuminate the subject. Similarly, fewer electrons penetrate a substrate when the electrons strike at an oblique angle. Most SEMs, however, are not able to tilt their subject. Furthermore, poor conductors, even when tilted, will charge under prolonged exposure. The water content in an organic resist is unfortunately decreased to a level which makes it ineffective in controlling charging after most patterning and developing processes. Baked or etched resists are further hardened such that virtually no water exists in or on the surface of the resist. Finally, adding electrolytes is undesirable because they change the etching properties of the resist.
Therefore, a need has arisen for an antistatic coating for integrated circuits which permits precise linewidth measuring by a SEM, which does not require modification to existing SEMs to be compatible therewith, and which does not interact with the wafer chemistry during fabrication.